Because an organic EL device is a self-luminescent device, it is luminous, excellent in visibility, and capable of giving clear display, as compared with a liquid crystal device. Therefore, active investigations have been made.
C. W. Tang et al. of Eastman Kodak Company developed a two-layer type laminated structure element in 1987, and this enabled an organic EL device using an organic substance to be put into practical use. They laminated an electron transporting fluorescent substance and a hole transporting organic substance, and injected both charges in a layer of the fluorescent substance to make the layer emit, thereby making it possible to attain high luminance of 1,000 cd/m2 or more at a voltage of 10V or lower (for example, see Patent Document 1 and Patent Document 2).
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3194657
From differences in process of device preparation and characteristics of materials, organic EL devices are classified into a deposition type device using a low molecular weight material and a coating type device mainly using a high molecular weight material.
The deposition type device requires a vacuum deposition apparatus for film formation. The coating type device easily conducts film formation by applying a coating liquid to a substrate, and removing a solvent in the coating liquid, so that the production process is simple, and the production can be carried out at low cost. Coating can be conducted with an ink jet method, a spray coating method or a printing method, and expensive facilities are not necessary for the production.
General materials used in the preparation of the coating type device were polymer materials such as poly(1,4-phenylenevinylene) (hereinafter referred to as PPV) (for example, see Non-Patent Document 1).
Non-Patent Document 1: Applied Physics Letters 71-1, page 34 (1997)
An organic EL device is investigated in which the role of two layers is further finely divided, and separately from an emission layer, a hole injecting layer, a hole transporting layer and an electron transporting layer are provided. As hole injecting or transporting materials for the preparation of the hole injecting layer or the hole transporting layer by coating, poly(ethylenedioxythiophene)/poly(styrenesulfonate) (hereinafter referred to as PEDOT/PSS) is widely used (for example, see Non-Patent Document 2).
Non-Patent Document 2: optical Materials 9 (1998) 125
However, the PEDOT/PSS coating liquid is an aqueous gel dispersion hydrated with PSS, to which the molecular chain of PEDOT gives ionic interaction, and is therefore an acidic aqueous solution. For this reason, there is the difficulty on use such that the coating liquid corrodes a coating or printing apparatus such as an ink jet ejection head.
Further, it is pointed out that PSS in the coating film adversely affects an anode, and water used in the coating liquid remains in the device, leading to deterioration during driving. Additionally, it is said that a thiophene ring of PEDOT is reduced by electron influx. Due to those difficulties, it is not considered that PEDOT/PSS is a sufficient hole injecting and transporting material, and satisfactory device characteristics, particularly in durability, were not obtained.
On the other hand, as a hole injecting and transporting material in the deposition type device, copper phthalocyanine, and MTDATA represented by the following formula:
or its derivative (for example, see Patent Document 3) are proposed, but those cannot form a stable thin film by coating.
Patent Document: JP-A-4-308688
Further, to enhance durability of the organic EL device, it is considered to be good to use a compound having good thin film stability. Compounds having higher amorphous property gives higher thin film stability, and a glass transition point (Tg) is used as a measure of the amorphous property (for example, see Non-Patent Document 3).
Non-Patent Document 3: M & BE Association, Vol. 11, No. 1, pages 32-41, (2000), The Japan Society of Applied Physics.
It is considered that higher glass transition point (Tg) is better. However, glass transition point of MTDATA is 76° C., and it is not said that its amorphous property is high. Due to this, satisfactory device characteristics were not obtained in durability of the organic EL device and also in emission efficiency due to hole injecting and transporting characteristics.